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Study of CO2 and H2O adsorption competition in the
combined dry / steam reforming of biogas
Nolven Guilhaume, Daniel Bianchi, Ramaniya Wandawa, Wang Yin, Yves
Schuurman
To cite this version:
Study of CO2 and H2O adsorption competition in the combined dry / steam reforming of
biogas
Nolven Guilhaume1*, Daniel Bianchi1,Ramaniya A. Wandawa1, Wang Yin2, Yves
Schuurman1
1 Univ Lyon; IRCELYON-CNRS, 2 Avenue Albert Einstein, F-69626 Villeurbanne Cedex, France 2 Present address: Rijksuniversiteit Groningen, Department of Chemical Engineering, Nijenborgh 4,
9747 AG, Groningen, the Netherlands
*Corresponding author (Nolven.Guilhaume@ircelyon.univ-lyon1.fr, +33-472445389)
Keywords: Biogas reforming; steam reforming; dry reforming; CO2 / H2O adsorption
competition.
Abstract
The combined dry-steam reforming (CDSR) of a model biogas (66% CH4 + 34% CO2) has
been investigated under various feed compositions and reaction temperature conditions over a Ni-Rh/MgAl2O4 catalyst, with the objective to convert the excess of methane (compared to
CO2) by steam reforming (SR). Methane, however, appeared to react preferentially with H2O
rather than with CO2, leading to an inhibition of the dry reforming (DR) reaction in the
presence of steam. CO2 and H2O adsorption and co-adsorption studies revealed that H2O was
always adsorbed in much higher amounts than CO2 on the catalyst surface. A Temkin
adsorption model applied to temperature-programmed adsorption experiments (TPAE) showed that two different strongly adsorbed CO2 and H2O species were present on the
catalyst surface at moderate temperature, but only one of each remained adsorbed at temperatures relevant to the reaction conditions (600-800°C). H2O was also shown to displace
CO2 from the catalyst surface, confirming that they compete, at least partially, for the same
adsorption sites. At high temperature, the surface coverage by the most strongly adsorbed H2O species was predominant. These results should contribute to the development of efficient
Introduction
Biogas is formed by microbial anaerobic digestion of biomass and contains essentially methane and carbon dioxide. Therefore, methane dry-reforming (DR, Eq. 1) is clearly the most advantageous option, in terms of CO2 emissions mitigation, to convert biogas into
syngas for hydrogen production or for Fischer-Tropsch synthesis. It can be reformed into syngas over Ni-based catalysts [1]. However, the CO2/CH4 ratio in biogas is generally <1, and
full conversion of methane requires adding another reactant. It might ideally be obtained by combining steam reforming (SR, Eq. 2) and dry reforming reactions, which also allows tuning the outlet H2/CO ratio.
CH4 + CO2 ➞ 2CO + 2H2 (DH0298K= 247 kJ/mol) (Eq. 1)
CH4 + H2O ➞ CO + 3H2 (DH0298K= 206 kJ/mol) (Eq. 2)
CO + H2O ⇄ CO2 + H2 (DH0298K= -41 kJ/mol) (Eq. 3)
The reforming reactions are always accompanied by the WGS/RWGS equilibrium (Eq. 3), which is displaced towards the RWGS reaction at high temperature [2, 3].
The combined dry-steam reforming (CDSR) has been investigated by Noronha et al. over Pt/ZrO2 and Pt/Ce-ZrO2 catalysts, from the standpoint of catalyst stability and coke deposition
[4]. Addition of steam in the feed initially increased the CH4 conversion over Pt/ZrO2, but the
catalyst deactivated more rapidly than under pure DR conditions, whereas the Pt/Ce-ZrO2
catalyst was more stable in the presence of steam. Surprisingly, more coke deposits were formed on both catalysts under CDSR than under DR. In contrast, addition of small amounts of steam in a DR stoichiometric mixture has been reported to prevent the deactivation of a Ni-based catalyst by reducing coke formation [5]. However the effect of steam addition in a stoichiometric CH4/CO2 mixture on the outlet composition (H2/CO ratio) was not reported nor
investigated in details.
Wei and Iglesia [6] studied independently the kinetics of the steam and dry methane reforming reactions on Ni/MgO catalysts. They concluded that the methane C-H bond activation was the only kinetically relevant step, whereas CO2 and H2O activation was
reversible and quasi-equilibrated during CH4/CO2 and CH4/H2O reactions. The forward CH4
reaction rate constants and activation energies were identical using CO2 or H2O as
proposed: methane dissociates through cascade steps into adsorbed C* and H*. Theoretical calculations suggest that adsorbed C* reacts preferentially with adsorbed HO* groups to form CO and H2, rather than with adsorbed O* species [7]. HO* species are formed directly from
H2O dissociative adsorption in the steam reforming reaction, whereas they are formed by
reaction of CO2 with H* in the case of dry-reforming.
CH4 + * → C* + 4H* (Eq. 4)
SR: H2O ⇄ H* + HO* (Eq. 5)
DR: CO2 + H* ⇄ CO + HO* (Eq. 6)
C* + HO* → CO + H* (Eq. 7)
2H* → H2 + 2* (Eq. 8)
The water-gas shift reactions are generally at thermodynamic equilibrium over a wide range of temperatures and reaction conditions [7, 6].
The adsorption competition between CO2 and H2O in relation to the dry/steam reactions has
been scarcely studied. It has been more intensively studied from the viewpoint of CO2
separation and capture (therefore performed at low temperature compared to those relevant to DR and SR reactions) using various porous materials, notably MOFs and organic polymers, but a few studies on zeolites are also reported. Stevens et al. [8] showed that a 13X zeolite exhibited the highest CO2 adsorption capacity among five different zeolites, but that exposure
of 13X to H2O before CO2 totally blocked the CO2 adsorption sites, since no adsorbed CO2
species were detected. CO2 and H2O co-adsorption experiments also revealed that H2O could
displace adsorbed CO2 species under a CO2 partial pressure of 1013 Pa, indicating that CO2
and H2O compete for the same adsorption sites. The study of Wang and LeVan [9] on the
adsorption equilibrium of CO2/H2O mixtures in zeolites 5A and 13X also concluded to a
competitive adsorption in favor of H2O over weakly adsorbed CO2 species, but also pointed
out that increasing the water partial pressure and temperature promotes CO2 chemisorption as
carbonates or bicarbonates. Ohlin et al. [10] investigated the effect of H2O on the CH4 and
CO2 adsorption properties of a Na-ZSM-5 zeolite with a high Si/Al ratio (Si/Al=130, close to
pure silica MFI), in view of application to CO2 separation from CH4 in biogas. H2O was
adsorbed in higher amounts than both CH4 and CO2, even when the CO2 partial pressure was
47 times higher than that of H2O. In this essentially silica framework zeolite, however, CO2
hydrophobic sites of the pore walls) was detected at 35°C, whereas H2O adsorbed on polar
silanol groups, therefore the adsorption was not competitive. In these studies, since CO2
separation from gas mixtures is sought out, adsorption studies were performed at room or at low temperatures and involved mostly weakly adsorbed CO2 species, which are not relevant
to the species involved in the dry reforming reaction.
In the present study, the performances of a Ni-Rh/MgAl2O4 catalyst for DR, SR and CDSR
were investigated under various reaction feed and temperature conditions, with the objective to assess the effect of steam on the DR reaction. This catalyst was previously studied in the autothermal reforming of model biogas to produce syngas, and it exhibited long-term performances (over 200 h) and resistance to carbon deposition and Ni oxidation [11, 12]. However, in the autothermal reforming reaction CH4 reacts only with O2 and H2O, while CO2
present in the biogas is not utilized. Therefore, this study addressed specifically the reforming of biogas into syngas through the DR and CDSR reactions. The CO2 and H2O adsorption and
co-adsorption properties were studied using transient experiments and reveal an adsorption competition between H2O and CO2 that leads to an inhibition of the DR reaction by steam.
Materials and methods
Catalyst preparation
The MgAl2O4 support was prepared by co-precipitation. An aqueous solution containing Mg
and Al nitrates was drip-fed to an aqueous solution of ammonium carbonate in large excess (100 %) at room temperature under strong stirring. This excess was necessary to ensure complete precipitation of Mg cations [13]. The precipitate was aged for 3 h at 60°C and pH 9. After filtration and washing with de-ionized water at room temperature, the powder was dried overnight at 100°C, then calcined at 800°C for 5 h in air. Chemical analysis of the support (analyzed by ICP-AES): 33.45 wt.% Al, 14.78 wt.% Mg.
Ni was deposited on the MgAl2O4 support by deposition-precipitation using nickel nitrate and
urea in large excess (10 moles per mole Ni). The suspension was heated at 90°C for 4 h under stirring. After cooling, the precipitate was filtered, washed several times with de-ionized water, dried at 100°C and calcined at 550°C for 4 h in air. Rh was deposited on NiO/MgAl2O4
in air. Chemical analysis of final catalyst: 10.21 wt.% Ni, 0.05 wt.% Rh. BET surface area: 160 m2.g-1.
TEM micrographs (not shown) of fresh 10%Ni-0.05%Rh/MgAl2O4 catalyst, after reduction
under 20% H2 at 700°C, showed that Ni was homogeneously distributed on the support as
rounded Ni particles in the 4-10 nm size range, with an average size around 6 nm. Rh could not be observed due to the low Rh loading.
Reaction studies
Dry-reforming (DR) and combined dry-steam reforming (CDSR) catalytic tests were performed at atmospheric pressure using a quartz fixed bed reactor. The catalyst was sieved beforehand to obtain particle sizes between 100 and 200 microns. The catalyst mass used was 80 mg, diluted with quartz powder (160 mg) with a similar particle size. A K-type thermocouple was inserted at the bottom of the catalyst bed, close to the quartz frit, to monitor the catalyst temperature. Before reaction, the catalyst was reduced at 700°C for 3 hours in a mixture of H2 (20%) in Ar. Catalytic tests were performed isothermally with a total reactants
flow rate of 250 mL/min, which corresponded to a WHSV of 188 L.h-1.g-1. The typical DR
feed composition consisted of 10% CH4, 10% CO2 and 10% He diluted in Ar. Helium was
used as internal standard for quantitative analysis of products, because the effluent flow rate was higher than the inlet flow rate, due to the net gas production resulting from the reforming reactions stoichiometry (Eq. 1 & 2). In SR and CDSR experiments, the liquid water flow rate was controlled by a calibrated HPLC pump (Shimadzu LC-20AD, liquid flow range 0.0001-5 mL/min) and dispensed through an evaporator heated at 200°C. All gas lines and valves were heated at 130°C to avoid steam condensation. The effluent from the reactor was analyzed online using a micro-gas chromatograph (Agilent 3000) equipped with two analytical modules (Plot U and MolSieve 5Å columns with backflush) and TCD detectors, that allowed analyzing CH4, CO2, CO, H2 and He. Steam was condensed in a cold trap at 5°C at the reactor outlet to
prevent any liquid condensation in the chromatograph micro-fluidic injection valves. The carbon balance was equal to 100 ± 0.02 % in all experiments. The HSC-Chemistry software version 4 (OUTOKUMPU, Finland) was used to calculate the equilibrium compositions, based on the minimization of the free Gibbs energy. It should be mentioned that the calculations were based on the presence of CH4, CO, CO2, H2, H2O and Ar in the gas phase,
the carbon balance with only CH4, CO and CO2. The catalyst composition, with its basic
MgAl2O4 support and Rh as dopant, has been shown to exhibit long-term activity for methane
autothermal reforming with marginal carbon deposition [11, 12].
Adsorption studies
CO2 and H2O adsorption studies were performed on a bench equipped with two gas lines and
two automated switching 4-way valves for transient experiments. The gases (CO2, Ar as tracer
and He as diluent) were supplied through calibrated mass flow controllers, with a total flow rate of 100 mL/min. Water was introduced using a saturator/condenser system where the gas bubbled first in two liquid water baths at 25°C in series, before bubbling in a third vessel immersed in a thermostated bath at cooler temperature (5-15°C), where the excess steam was condensed to ensure full saturation of the gas. The temperature of the condenser was regulated with an accuracy of ±0.1°C. After the saturator, all gas lines and valves were heated at 130°C to avoid steam condensation. Gas analysis was monitored using a mass spectrometer (Pfeiffer Omnistar) with heated capillary and inlet assembly. Around 0.20 g of catalyst was placed in a U-shape reactor for adsorption experiments, with a thin thermocouple placed inside the catalyst bed to monitor the catalyst temperature. The catalyst was initially reduced at 700°C under 20% H2 in He for 1 h, then desorbed and re-reduced at 500°C between each adsorption
measurement. A typical experiment involved switching the gas composition according to the sequence He → 2% CO2 /2% Ar/He, repeated 3 times to provide in the first sequence the total
amount of strongly and reversibly adsorbed CO2 species, then in the next 2 sequences the
amount of reversibly adsorbed species, the third one used to confirm the results of the second. After the adsorption equilibrium at a selected temperature, the temperature was increased linearly at 15°C/min in the presence of CO2 to perform a temperature-programmed adsorption
equilibrium (TPAE) experiment [14], or under pure He to perform a classic temperature-programmed desorption (TPD) of strongly adsorbed species. Similar experiments were carried out with H2O/Ar/He and CO2/H2O/Ar/He gas mixtures.
Results and discussion
Effect of steam addition on dry-reforming catalyst performances
The 10%Ni-0.05%Rh/MgAl2O4 catalyst was tested under DR and CDSR conditions at
different temperatures (Fig. 1). Under DR conditions (H2O/CH4 = 0) the CO2/CH4 ratio was
the H2/CO outlet ratio was 0.73 at 487°C and increased with temperature up to 0.92 at 670°C.
The conversion of CO2 was always higher than the conversion of CH4, due to the contribution
of the RWGS reaction that consumes CO2 and H2 and results in a H2/CO ratio less than unity
[7]. The conversions of CH4 and CO2 were always below the expected conversions at the
thermodynamic equilibrium, due to the high WHSV (188 L.h-1.g-1) that resulted in a low
contact time (0.019 g.s.mL-1). Verykios [15] showed that the DR of a CH4/CO2 mixture with
similar composition over a Ni/La2O3 catalyst reached equilibrium compositions only at
contact times of 0.06 g.s.mL-1 and above.
Under CDSR conditions, the CO2/CH4 ratio was set to 0.66 (10 vol.% CH4 and 6.60 vol.%
CO2), therefore adding steam in the reactants mixture with a H2O/CH4 ratio of 0.34 represents
a stoichiometric CO2:H2O:CH4 0.66:0.34:1 mixture, considering that methane is converted by
both dry and steam reforming reactions (Eq. 1 & 2). Adding steam in the feed stream led to an increase in the conversion of methane at H2O/CH4 ratios of 1 and above (Fig. 1A), and to a
strong increase in the H2/CO ratio (Fig. 1C), as the SR and WGS reactions became favored.
The contribution of the WGS reaction was particularly strong at 487°C, since it is thermodynamically favored at low temperature and at high H2O/CH4 ratio, leading to an
outlet H2/CO ratio close to 10 at 487°C for an inlet H2O/CH4 ratio of 2, while the H2/CO ratio
expected from the sole SR reaction (Eq. 2) is 3. The H2/CO ratios were generally close to or
slightly lower than the calculated equilibrium ratios (Fig. 1C), probably due to slight differences between the experimental conditions (temperature, feed composition) and the conditions defined for the calculations. In contrast with methane, the conversion of CO2 (Fig.
1B) was strongly affected by the presence of steam and decreased steadily with increasing H2O/CH4 ratios. At 487°C and H2O/CH4 ≥ 0.68, the conversion of CO2 became apparently
negative, since more CO2 was found at the reactor outlet than in the inlet feed, as the result of
CO conversion into CO2 by the WGS reaction. At H2O/CH4 = 2 the conversion of CO2 was
(A)
(B)
Fig. 1: 10%Ni-0.05%Rh/MgAl2O4 catalyst performances for DR and CDSR at different
temperatures and H2O/CH4 ratios. Circles: equilibrium values. (A) CH4 conversion; (B) CO2
conversion; (C) H2/CO ratio. Conditions: CO2/CH4 = 1 in the absence of H2O (DR);
CO2/CH4=0.66 in the presence of H2O (CDSR).
Table 1 summarizes the methane activities measured at 487°C under the different reactants compositions investigated. The rates are given only for the lowest temperature studied, in the low methane conversion range (15-32%), because at high temperature and conversions the reaction was subject to external diffusion limitations, as evidenced by the Arrhenius plots displayed in Fig. 2.
Table 1: CH4 reaction rates under DR and CDSR conditions at 487°C
CH4 reaction rate (mmol.s-1.g cat-1) DR 0.033 CDSR H2O/CH4=0.34 0.042 CDSR H2O/CH4=0.68 0.048 CDSR H2O/CH4=1 0.060 CDSR H2O/CH4=2 0.067
Effect of carbon dioxide addition on steam-reforming catalyst performances
Fig. 3 shows the effect of CO2 addition to a stoichiometric SR feed at H2O/CH4 = 1. The
methane reaction rate measured at 587°C under pure SR conditions was 0.083 mmol.s-1.g-1,
slightly slower than the CH4 reaction rate measured at 577°C under pure DR conditions
(0.103 mmol.s-1.g-1, Table 1). The methane conversion (Fig. 3A) was only slightly affected by
the introduction of CO2 in the feed, changing from 41.2% at CO2/CH4=0 to 46.5% at
CO2/CH4=1. These conversions were well below the calculated equilibrium conversions. In
contrast, the H2/CO ratio decreased strongly with CO2 addition in the feed, which displaces
the WGS equilibrium towards the RWGS reaction. Compared to methane, CO2 appeared very
moderately converted (7.4 and 11.9% at CO2/CH4=0.6 and CO2/CH4=1, respectively),
nevertheless this conversion can be attributed to both DR and RWGS reactions. In Fig. 3B, the H2 and CO production rates at CO2/CH4 = 0, 0.6 or 1 are plotted as a function of the
methane conversion reported in Fig. 3A. The CO production increased from 0.25 mmol/min at CO2/CH4 = 0 to 0.61 mmol/min at CO2/CH4 = 1 and the H2 production decreased from 1.44
mmol/min to 1.28 mmol/min, while the methane conversion increased by ≈ 5%. Therefore, the CO production rate increased by 0.36 mmol/min when the H2 production rate decreased
by 0.16 mmol/min, which suggests that the RWGS reaction cannot account by itself for the conversion of CO2 and that the DR reaction also contributes to the conversion of CO2. These
results, and particularly the reaction of the CO2:H2O:CH4 1:1:1 mixture in Fig. 3A, suggest
that methane (or more specifically adsorbed carbon formed by methane decomposition) reacts preferentially with H2O rather than with CO2 over 10%Ni-0.05%Rh/MgAl2O4, in spite of the
(A)
(B)
Fig. 3: 10%Ni-0.05%Rh/MgAl2O4 catalyst performances for SR and CDSR at different
CO2/CH4 inlet ratios. (A) Conversion of CH4 and CO2 and H2/CO outlet ratio; (B) H2 and CO
production versus CH4 conversion. Conditions: H2O/CH4 = 1 (SR and CDSR), CO2/CH4
variable (CDSR). Catalyst temperature: T= 587°C at CO2/CH4 = 0; T=584°C at
CO2/CH4=0.6; T=583°C at CO2/CH4=1.
In the experiments displayed in Fig. 3, the temperature measured in the catalyst bed was 600°C when the reactor was swept with Ar. Switching to the reaction conditions led to an immediate drop in the catalyst temperature, which translates the reforming reactions endothermicity (Eq. 1 and 2): under pure SR conditions, the catalyst temperature decreased to 587°C, and to 584 and 583°C under CSDR with CO2/CH4 = 0.6 and 1, respectively, in
supports the contribution of the DR reaction, despite its limited extent, under these conditions (H2O/CH4=1). With the same catalyst temperature of 600°C under Ar, the catalyst
temperature dropped to 577°C under the pure DR conditions shown in Fig. 1.
CO2 adsorption studies
CO2 undergoes dissociative chemisorption on Ni and on Rh, with formation of anionic CO2
-species through electron transfer [2]. On Ni, the chemisorption properties were also shown to depend on the type of surface planes considered [16]. However, due to its acidic character, CO2 also interacts strongly with the basic MgAl2O4 support. IR [17] and DRIFTS studies of
CO2 adsorption on MgxAlyOz mixed oxides with Mg/Al= 0.1-0.3 [18] evidenced the presence
of 3 types of adsorbed species: bicarbonates HCO3- that form with surface OH groups,
bidentate carbonates bridging Mg2+ and O2- ions, and monodentate carbonates chemisorbed
on O2-. The bicarbonate species desorbed below 100°C, whereas both bidentate and
unidentate carbonates were present up to 300°C, but only unidentate carbonates were detected above 350°C [17]. CO2 adsorption on Ni/MgAl2O4 or on Rh/MgAl2O4 led to similar adsorbed
species [19, 20]. Basic supports such as La2O3 and MgAlOx mixed oxides derived from
hydrotalcites have been proposed to participate in the DR reaction mechanism through the formation of surface carbonate species that react with adsorbed carbon formed by decomposition of methane or propane [20, 21, 22].
The adsorption properties of the 10%Ni-0.05%Rh/MgAl2O4 catalyst for CO2 were studied in
transient experiments in which diluted CO2 was adsorbed on the catalyst at different
temperatures and partial pressures. Fig. 4 displays an example of sequence in which the gas composition was switched according to: He → 2% CO2 / 2% Ar / He (Fig. 4A), followed by
2% CO2 / 2% Ar / He → He (Fig. 4B) and again He → 2% CO2 / 2% Ar / He (Fig. 4C). Ar
was used as tracer since it does not adsorb on the catalyst, and the area between the CO2 and
Ar signals represents the amounts of CO2 adsorbed or desorbed. The difference between total
and reversible CO2 gives the amount of strongly adsorbed species at the applied
Fig. 4: Adsorption / desorption sequence of 2% CO2/2% Ar/He on 10%Ni-0.05%Rh/MgAl2O4
catalyst at 250°C. (A) Total CO2 adsorbed; (B) CO2 reversibly desorbed; (C) CO2 reversibly
adsorbed.
Adsorption of CO2 at room temperature followed by TPAE also revealed the presence of both
weakly and strongly adsorbed CO2 species. The inset in Fig. 5 shows the initial CO2
-adsorption/He-desorption sequence at 25°C, in which 188 µmol/g of CO2 adsorb (total CO2)
while 64 µmol/g desorb under He (weakly adsorbed species). After re-adsorption of CO2 at
25°C, Fig. 5 shows the evolution of CO2 and Ar molar fractions at increasing adsorption
temperatures under quasi-equilibrium conditions (TPAE procedure, [14]). The poorly resolved CO2 peaks below 130°C (total amount 68 µmol/g) are due to the decrease in the
adsorption equilibrium coverage of the weakly adsorbed species, which are not relevant to the reaction conditions. The broad CO2 peak between 140 and 500 °C (≈103 µmol/g) is due to the
net desorption rate of the strongly adsorbed CO2 species. The final switch 2% CO2/2% Ar/He
➞ He (not shown) at 500°C leads to the desorption of ≈23 µmol/g of CO2, indicating that there
is a coverage in CO2 species at 500°C under PCO2= 2 kPa. The total amount of strongly
Fig. 5: TPAE on 10%Ni-0.05%Rh/MgAl2O4 under 2% CO2/2% Ar/He in the 25-500°C
adsorption temperature range. Inset: Initial adsorption of 2% CO2/2% Ar/He followed by
desorption under He at 25°C.
The CO2 peak at T>140°C during the TPAE provides the experimental average coverage of
the strongly adsorbed species (black circles in Fig. 6), considering that they are present at full coverage at T=140°C under PCO2= 2 kPa (see supplementary information), since the Ar and
CO2 molar fractions are equal at 130-140°C (Fig. 5). This experimental curve can be fitted
according to a modeling procedure described previously [23 and supplementary information], based on (a) the Temkin adsorption model, which assumes a linear decrease in the heat of adsorption when the surface coverage increases and (b) localized adsorbed species for the expression of the adsorption coefficient. The experimental data could not be modeled if only one CO2 species was considered, but they were adequately fitted (Fig. 6, curve a) assuming
two CO2 adsorbed species A and B, with different heats of adsorption and ratios at saturation
(Table 2). Curves b and c provide the individual coverages of the two CO2 species under
PCO2= 2kPa, and show that only species B is present on the catalyst surface in the temperature
Fig. 6: Coverage of strongly adsorbed CO2 species as a function of the adsorption temperature
under PCO2= 2 kPa. Black circles: experimental data; red line: model fit assuming two
different adsorbed species A and B; blue and magenta lines: individual calculated coverages of the two species A and B.
Table 2: Heats of adsorption of CO2 and H2O strongly adsorbed species on
10%Ni-0.05%Rh/MgAl2O4 catalyst at full and zero coverage, and ratio of each species at surface
saturation. EA (q=1) kJ/mol EA (q=0) kJ/mol Ratio at saturation CO2 species A 75 108 0.75 CO2 species B 110 180 0.25 H2O species A 78 105 0.55 H2O species B 105 173 0.45 H2O adsorption studies
Ni surfaces have been shown to display different reactivities towards water: H2O adsorbs
reversibly on relatively unreactive closed-packed (111) and (100) surfaces, whereas it can chemisorb or fully dissociate, depending on the conditions, on more open (110) surfaces and on stepped surfaces [24]. Water adsorption on alumina and on titania is generally considered non-dissociative and leads to the formation of similar adsorbed species: weakly physisorbed species and more strongly bound species chemisorbed on surface OH groups or on basic O-2
anions through hydrogen bonds, or coordinated to Al3+ or Ti4+ Lewis acid sites through the
oxygen atom [25, 26].
Fig. 7 displays an adsorption/desorption sequence with H2O at 250°C, in which the gas
composition was switched according to: He → 1.8% H2O / 2% Ar / He (Fig. 7A), followed by
1.8% H2O / 2% Ar / He → He (Fig. 7B) and He → 1.8% H2O / 2% Ar / He (Fig. 7C). The
amounts of steam adsorbed were significantly higher than those of CO2, and the rate of water
desorption decreased very slowly during the desorption. Moreover, a small H2 peak was
observed at the first introduction of steam on the catalyst (Supplementary data, Fig. 1), which was formed by dissociation of H2O on surface metallic Ni sites.
Fig. 7: Adsorption / desorption sequence of 1.8% H2O/2% Ar/He on
10%Ni-0.05%Rh/MgAl2O4 catalyst at 250°C. (A) Total H2O adsorbed; (B) H2O reversibly desorbed;
(C) H2O reversibly adsorbed.
Fig. 8 shows a TPAE performed on the reduced 10%Ni-0.05%Rh/MgAl2O4 catalyst under
0.87% H2O/0.87% Ar/He, where the initial adsorption temperature was set to 150°C to
prevent the contribution of weakly adsorbed H2O species. The total amount of water adsorbed
Fig. 8: TPAE on 10%Ni-0.05%Rh/MgAl2O4 under 0.87% H2O/0.87% Ar/He in the
150-500°C adsorption temperature range.
The temperature increase leads to a broad H2O peak, which corresponds to the decrease in the
adsorption equilibrium coverages of strongly adsorbed species. The total amount was 240 µmol H2O/g at Ta= 515°C, indicating that 290-240 = 50 µmol/g remained adsorbed at this
temperature. Assuming full coverage of the H2O species at 150°C, Fig. 9 provides the
evolution of the experimental adsorption equilibrium coverage with the increase in the adsorption temperature under quasi-isobaric conditions. Again, the Temkin adsorption model described the data adequately if two species were considered (Fig. 9, curve a). Since H2O
adsorption experiments revealed, through the formation of hydrogen, that a small fraction of water dissociated on reduced Ni, these two species were assumed to be molecular and dissociated H2O species. The heats of adsorption of the two species at high and low coverage
Fig. 9: Coverage of strongly adsorbed H2O species as a function of temperature at PH2O= 0.87
kPa. Black circles: experimental data; red: model fit assuming two different adsorbed species; blue and magenta: calculated individual coverages of molecularly (A) and dissociatively (B) adsorbed species.
CO2 and H2O competitive adsorption
Fig. 10 compares the amounts of total (QT) and strongly (QS) adsorbed CO2 and H2O at 150,
250 and 350°C. Whatever the temperature, QT(H2O) was roughly 3 times higher than
QT(CO2), while QS(H2O) was 2.5 to 4.5 times higher than QS(CO2). At similar adsorption
pressures, the adsorption of water is clearly favored over that of CO2. This can be due in part
to a difference in the nature of some adsorption sites for CO2 and H2O.
Fig. 10: Amounts of CO2 and H2O adsorbed at 150-350°C on 10%Ni-0.05%Rh/MgAl2O4.
The competitive CO2-H2O adsorption was evidenced through the sequence displayed in Fig.
11. CO2 was first adsorbed alone at 250°C, then H2O was added in the gas mixture and
subsequently removed. At each H2O addition, a CO2 desorption peak corresponding to 45 ± 5
mmol/g was observed while H2O adsorbed on the catalyst, i.e. before the onset of the H2O
signal. These results might involve both CO2 and H2O strongly adsorbed species evidenced
previously, however at higher temperature the detectable amounts of CO2 displaced by H2O
decrease rapidly.
Fig. 11: Transient response to successive gas switches 2% CO2/2% Ar /He → 2% CO2/1.8%
H2O /2% Ar /He over 10%Ni-0.05%Rh/MgAl2O4 catalyst at 250°C.
A deeper insight in the competitive adsorption between CO2 and H2O adsorbed species
relevant to the DR and CDSR reactions was obtained using a competitive Temkin model [27, 28] and the heats of adsorption calculated for CO2-B and for dissociatively adsorbed H2O-B
species (present on the catalyst surface in the 600-800°C temperature range). Fig. 12 shows the calculated evolution of the surface coverage of each species with temperature, considering the adsorption of single H2O or CO2 species (no competition) or their competitive
Fig. 12: Comparison of the calculated coverages of CO2-B and H2O-B species in the absence
of competition (dotted lines) or in competitive co-adsorption (full lines) at a CO2 and H2O
partial pressure of 2 kPa.
The model predicts that the CO2-B coverage strongly prevails up to 500°C, but at T>600°C it
decreases faster than the H2O-B coverage and the surface becomes predominantly covered by
dissociated H2O-B species. Although this result is calculated and should be confirmed by
direct identification of the different CO2 and H2O adsorbed species, i.e. using IR
spectroscopy, it provides a plausible explanation to the catalyst behavior observed in CDSR experiments.
Conclusion
Combined dry-steam methane reforming was applied to convert a model biogas into syn-gas over a 10%Ni-0.05%Rh/MgAl2O4 catalyst. The effect of CO2 addition to a steam reforming
feed steam, and conversely the effect of H2O addition to a dry reforming feed stream,
evidenced an adsorption competition between CO2 and H2O that led to an inhibition of the
dry-reforming reaction in the presence of steam. CO2 and H2O adsorption studies at different
temperatures showed that H2O was always adsorbed in much higher amounts than CO2 on the
catalyst surface. A Temkin-based model applied to quasi-equilibrium adsorption data showed that two different strongly adsorbed CO2 and H2O species were present on the catalyst surface
surface, confirming that they compete, at least partially, for the same adsorption sites. At high temperature, the surface coverage by the most strongly adsorbed H2O species was
predominant.
Acknowledgements
This project has received funding from the European Union’s Horizon 2020 research and innovation programme under grant agreement No 736272.
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